In general, a DC motor comprises a plurality of interpoles in order to improve the commutating characteristics of the DC motor. The interpoles are disposed at the intermediate position along the outer circumference of an armature between the adjacent main magnetic poles, and face the armature windings existing in commutating zones. The interpoles are interlinked by a magnetic field produced by the armature reactions, and accordingly the interpoles are necessary to produce a magnetomotive force to eliminate the magnetic field produced by the armature reactions, in addition to a magnetomotive force to produce a magnetic field needed for commutation.
In order to reduce the magnetomotive force produced by the interpoles and to improve various operating characteristics of the DC motor, a DC motor having E-shaped interpoles has been proposed in, for example, the Japanese Patent Application Laid Open No. 53-126107 corresponding to U.S. patent application Ser. No. 884,586, now U.S. Pat. No. 4,220,882 by the applicant of the present invention.
With reference to the accompanying FIG. 1, a prior art DC motor having the E-shaped interpoles disclosed in the above-mentioned patent application will be explained. The DC motor in FIG. 1 comprises an armature 1, a cylindrical shaped yoke 7 and two main magnetic poles 3 and 4 equidistantly spaced around the outer circumference of the armature 1 having a small gap therebetween and attached to the inner circumference of the yoke 7. The main magnetic poles 3 and 4 have field windings 5 and 6 wound thereon respectively. The field windings 5 and 6 are supplied with electric current in a predetermined direction so that the polarities of the main magnetic poles 3 and 4 are selected to be, for example, N and S, respectively. As a result, the armature 1 is counter clockwisely rotated as shown by arrow a. In this case, some of the armature windings 21, 22 and 23, 24, which are located between the main magnetic poles 3 and 4, are within commutating zones.
In order to eliminate the counter electromotive force induced in the armature windings 21 through 24 within commutating zones, the E-shaped interpoles 8 and 9 are attached to the inner circumference of the yoke 7 by using spacers 25 and 26 made of non-magnetic material, and are located at the intermediate positions along the outer circumference of the armature 1 between the main magnetic poles 3 and 4. The interpole 8 comprises a center pole 81 having an interpole winding 10 wound thereon, and two side poles 82 and 83 disposed respectively in front of and to the rear of the center pole 81 along the direction of the rotation of the armature 1. The interpole winding 10 is connected in series with the armature windings 2, and an armature current passes through the interpole winding 10 in such a direction so that the polarity of the center pole 81 becomes S and the polarity of the side poles 82 and 83 becomes N. The other interpole 9 also comprises a center pole 91 having an interpole winding 11 wound thereon, and two side poles 92 and 93. The interpole winding 11 is also connected in series with the armature windings 2, and the direction of the armature current passing through the interpole winding 11 is selected so that the polarity of the center pole 91 becomes N and the polarity of the side poles 92 and 93 becomes S.
The E-shaped interpoles 8 and 9 are hardly affected by the magnetic field produced by the armature reactions. This is ecause the magnetic flux caused by the whole of the armature current flowing through the armature windings 2 hardly penetrates the interpoles 8 and 9 due to the existence of the spacers 25 and 26 of non-magnetic material. For example, in the interpole 8, only the magnetic flux f.sub.1 and f.sub.2, which are caused by the current passing through the armature windings 21 and 22 within a commutating zone, pass through the magnetic circuit including the center pole 81, side poles 82 and 83 of the interpole 8 and the armature 1. Thus, the amount of the magnetomotive force produced by the E-shaped interpoles 8 and 9 can be greatly reduced, and therefore, the cross sectional area of the interpole winding can be very small and the heat generated by the interpoles can be reduced.
It should be noted that the amount of the magnetic flux produced by the interpole winding is desirable to be linear to the amount of the current flowing through the interpole winding, and if the linearity of the amount of the magnetic flux is not guaranteed in the wide range of the interpole current, the interpole cannot effectively perform its function of eliminating the armature reaction and producing the magnetic flux for commutation.
FIG. 2 shows the conventional E-shaped interpole used in the conventional DC motor of FIG. 1, and FIG. 4A shows a relation between the magnetic flux for commutation and the armature current. In the conventional E-shaped interpole of FIG. 2, among a plurality of teeth of the armature 1 which face the end surfaces of the side poles 82 and 83 of the interpole 8, the teeth, which are nearer to the center pole 81 of the interpole 8, become magnetically saturated easily with a smaller armature current than the other teeth. Therefore, it is impossible to supply a sufficiently large current to the interpole winding, and the linearity of the magnetic flux with the armature current cannot be obtained in the wide range of the armature current.
The reason why the above-mentioned linearity cannot be obtained in a wide range will be explained in more detail. In the magnetic circuit composed of the E-shaped interpole 8 and the armature 1 as shown in FIG. 2, the magnetic flux produced by the armature windings which face the E-shaped interpole 8 is added to the magnetic flux produced by the interpole winding 10 in an opposite direction. The magnetic flux produced by the interpole winding 10 is larger than that produced by the armature windings facing the E-shaped interpole 8, and consequently the magnetic flux for commutation is produced in the same direction as that of the magnetic flux produced by the interpole winding 10, as shown by arrows in FIG. 2. This magnetic flux for commutation passes the magnetic circuit composed of the E-shaped interpole 8 and the armature 1 from the center pole 81 through both side poles 82, 83 and the armature 1 to the center pole 81 again. In this case, the magnetic flux for commutation passes the above-mentioned magnetic circuit separately into the left half and the right half thereof. Half of the magnetic flux for commutation which passes, for example, the right half of the above-mentioned magnetic circuit is composed of the magnetic flux f.sub.A which passes the magnetic circuit P.sub.A including the tooth A of the armature 1 and the side pole 83 and the center pole 81 of the E-shaped interpole 8, and the magnetic flux f.sub.B which passes the magnetic circuit P.sub.B including the tooth B of the armature 1 and the side pole 83 and the center pole 81 of the E-shaped interpole 8. It should be noted that the tooth A faces the portion of the end surface of the side pole 83 which is adjacent to the center pole 81, and the tooth B faces the portion of the end surface of the side pole 83 which is remote from the center pole 81. As mentioned above, the amount of the magnetic flux for commutation is determined by the difference between the amount of the magnetic flux produced by the interpole winding 10 and the amount of the magnetic flux produced by the armature windings 2 facing the E-shaped interpole 8. Since the number of the armature windings 2 which interlink with the magnetic circuit P.sub.A is smaller than the number of the armature windings 2 which interlink with the magnetic circuit P.sub.B , the amount of the magnetic flux which passes through the magnetic circuit P.sub.B and produced by the armature windings 2 is larger than the amount of the magnetic flux which passes through the magnetic circuit P.sub.A and produced by the armature windings 2. Therefore, the amount of the magnetic flux f.sub.A is larger than the amount of the magnetic flux f.sub.B , and the magnetic flux density in the tooth A is larger than that in the tooth B. As shown in FIG. 4A, the amounts of the magnetic flux f.sub.A and f.sub.B increase according to the increase of the armature current I.sub.a , i.e. interpole current, and the tooth A is magnetically saturated prior to the magnetical saturation of the tooth B. To this end, the linearity of the total magnetic flux f.sub.A +f.sub.B is lost in accordance with the saturation of the magnetic flux f.sub.A , as shown by the bold line of FIG. 4A. As a result, the linearity of the amount of the magnetic flux for commutation cannot be guaranteed in a wide enough range of the interpole current.